Atomic layer deposition is a process commonly used to form thin films in the semiconductor industry. A typical process includes positioning a substrate in a reactor and providing a first precursor to the reactor. The first precursor deposits a first species on the substrate surface until the surface is saturated with the first species, after which deposition stops. A second precursor is then provided to the chamber. The second precursor reacts with the first species lining the surface of the substrate until no more of the first species is available to react, after which deposition stops. Such cycles are repeated until a desired thickness of the layer is formed. The chamber is typically purged between precursors to provide controlled layering on the substrate.
The atomic layer deposition process is useful for forming layers having very uniform thickness and composition because the deposition reaction is controlled at the molecular, or atomic, level. The first species only adheres to the substrate surface if an adhesion site is available. Every instance of the first species adheres to the substrate surface in exactly the same way, so that it can participate in the reaction with the second precursor in exactly the same way.
The layer deposited in each deposition operation is monomolecular or monatomic. Typically, the species deposited are no larger than small molecules. Thus, each layer deposited typically has a thickness of 5 Å or less. Each cycle consisting of two precursor operations and two purge operations may take up to a minute to execute. More complex cycles involving more than two precursors may take longer. Forming layers 50-100 Å thick may take 10-20 minutes. To improve rates in ALD processes, one or more precursors may be activated, for example by forming a plasma. The precursor is flowed into the chamber, and then plasma is formed to activate deposition. Plasma is typically discontinued when deposition from the precursor is complete. Fast cycling of gases promotes high throughput. Thus, there is a continuing need for apparatus and methods for fast cycling in PEALD processes.